Just like visible light, infrared light, and radio waves, ultraviolet light is electromagnetic radiation. Ultraviolet light lies on the spectrum between violet light and X-rays . Although ultraviolet radiation is undetectable to the naked eye, anyone who has been exposed to too much sunlight has probably noted the effects of ultraviolet light. It is this form of radiation that causes tanning and sunburn, types of skin damage which can lead to skin cancer.
The man credited with the discovery of ultraviolet light is German physicist Johann Ritter (1776-1910). Ritter experimented with silver chloride, a chemical known to break down when exposed to sunlight. He found that the light at the blue end of the visible spectrum (blue, indigo, and violet) was a much more efficient stimulant for this reaction. Experimenting further, Ritter discovered that silver chloride broke down most efficiently when exposed to radiation just beyond the blues. He called this new type of radiation ultraviolet, meaning "beyond the violet."
Ultraviolet Radiation and the Body
While ultraviolet radiation in large doses is hazardous to humans, a certain amount is actually required by the body. As it strikes the skin, ultraviolet rays activate the chemical processes that produce vitamin D. In areas that lack adequate sunshine, children are often plagued by rickets (a disease characterized by abnormally shaped and structured bones). In order to treat these cases, or to supplement natural light in sun-starved communities, ultraviolet lamps are often used.
Three Types of Lamps
There are three varieties of ultraviolet lamps, each producing ultraviolet light of a different intensity. Near-ultraviolet lamps are fluorescent lights whose visible light has been blocked, releasing ultraviolet radiation just beyond the visible spectrum. These lamps are also known as black lights, and are primarily used to make fluorescent paints and dyes "glow" in the dark. This effect is often used for entertainment, but can also be used by industry to detect flaws in machine parts.
Middle-ultraviolet lamps produce radiation of a slightly shorter wave-length. They generally employ an excited arc of mercury vapor and a specially designed glass bulb. Because middle-ultraviolet radiation is very similar to that produced by the sun, these lamps are frequently used as sunlamps. They are often found in tanning salons and greenhouses. Photochemical lamps generating middle-ultraviolet light are also used in chemical laboratories and industrial settings to induce certain chemical reactions.
Far-ultraviolet lamps produce high-energy, short-wavelength ultraviolet light. Like middle-ultraviolet lamps, they use mercury-vapor tubes. Far-ultraviolet radiation is easily absorbed by glass, so the lamp's bulb must be constructed from quartz. Far-ultraviolet light has been found to destroy living organisms such as germs and bacteria. These lamps are often used to sterilize hospital air and equipment. Far-ultraviolet radiation has also been used to kill bacteria in food and milk, giving perishables a much longer shelf life.